For several years Modicon's Modbus serial communications protocol has been the de facto standard for use in connecting industrial electronic devices. Modbus has been used in a vast variety of implementations where instructions must be sent from a supervisory or “master” station to a number of “slave” devices, wherein the values of memory registers associated with each slave device may be set or retrieved by the master station. Such configurations are commonly referred to as Supervisory Control and Data Acquisition (SCADA) systems. An illustrative Modbus implementation 70 is illustrated as a block diagram in FIG. 1, as contemplated by the relevant prior art.
In the illustrative Modbus implementation 70, a master station 72 is in electronic communication with one or more slave devices 71. The connection between the master station 70 and the slave devices 71, and optionally between additionally connected slave devices 71, is provided by a standard Modbus connection 73. Often, the standard Modbus connection 73 between the master station 72 and slave devices 71 is provided by a radio signal or a twisted pair cable. There are many limitations inherent in using these types of connections.
For example, the data transmission speed achievable for a typical Modbus system is inversely proportional to the length of the cable for the standard Modbus connection 73. As a result, the only way to achieve maximum data transmission rates of about 35 Mbps require the cable for the Modbus connection must be kept under 10 meters in length. Above 10 meters, the maximum possible transmission speed drops quickly as the length is increased, with 100 Kbps as the maximum achievable speed at 1,200 meters (i.e., less than 0.3% of the 10 meter rate). Also, since all data requests are initiated by the slave devices 71, current Modbus implementations of the prior art can only react to changes in register values that occur slower than the amount of time necessary to query all slave devices 71. This functional limitation, combined with the speed decreases inherent in longer cable lengths, make exception- or alarm-type event handling very difficult. As another limitation, the number of devices that can be attached to a typical Modbus system is limited to 247, based on the addressing standard dictated by the protocol.
Accordingly, the size and scope of the Modbus implementations of the prior art are severely restricted by the limitations of the currently available hardware, making the Modbus protocol an inappropriate vehicle for large-scale or municipal implementation. As a result, much time, effort, and expense have been committed to redesigning and rebuilding devices that can be used for larger scale systems, such as SCADA systems. Given the vast number of industrial devices and sensors that are already configured to work with the Modbus protocol, however, it would be advantageous to have a solution that would function with existing devices, while improving the capabilities of the data transmission network.
Other inventors have combined Modbus technology with newer network devices in an attempt to overcome some of these limitations. These solutions, however, only provide the ability to connect multiple Modbus networks to one or more computing devices using a Local Area Network (LAN), Wide Area Network (WAN), or the like. These attempts to address the shortcomings of Modbus technology do not effectively overcome limitations associated with data query speeds or enable preferred event handling models, such as exception- or alarm-based ones.